An FBAR (Film Bulk Acoustic Wave Resonator) and an SMR (Solidly Mounted Resonator) are well known as thin film piezoelectric bulk acoustic wave resonators suitable for a radio frequency filter.
U.S. Pat. No. 6,617,249B2 discloses an FBAR-type thin film bulk acoustic wave resonator that allows resonators with different resonant frequencies to be fabricated on the same substrate by forming a surface loading layer, which covers a surface electrode layer and is patterned thereon, on the surface electrode layer.
U.S. Pat. No. 5,912,601 discloses a piezoelectric resonator composed of a laminate that is formed by a plurality of laminated piezoelectric layers between which a substrate is interposed that has a plurality of electrodes with different areas.
“Aigner et al., Bulk-Acoustic Wave Filters: Performance Optimization and Volume Manufacturing, 2003 IEEE MTT-S Digest, pp. 2001-2004” describes an SMR-type thin film bulk acoustic filter that shifts resonant frequency by adding an auxiliary metal layer on top of a surface electrode layer, in order to fabricate a resonator with a different frequency.
“Wang et al., Method of Fabricating Multiple-frequency Film Bulk Acoustic Resonator (FBARs) in a Single Chip, 2006, IEEE Frequency Control Symposium Digest, p. 179” describes a method of enabling the adjustment of resonant frequency by providing an additional adjusting layer on top of a surface electrode layer and thereby controlling the width of the adjusting layer or pitch adjusting pattern.